Liquid crystal devices are frequently used for displays because of their ability to selectively transmit or block light depending on its polarization. The electro-optic effects of such devices also allow them to be used in shuttering applications where it is desired to pass or block light depending on the operational characteristics of the viewing system in which they are incorporated.
Liquid crystal devices composed of twisted nematic liquid crystal materials have received much attention as candidates for displays and shutters. These devices are produced by fabricating a liquid crystal cell and positioning it between optical polarizing filters. The cell contains a layer of nematic liquid crystal material sandwiched between a pair of parallel transparent plates.
The surfaces of the plates in contact with the liquid crystal material are coated in select areas with a transparent conducting material, forming an electrode on each surface. The surfaces of the electrodes in contact with the liquid crystal material are conditioned so that they impart a preferential alignment to the director axes of the material. If the plates are aligned so that their respective preferred directions are orthogonal, the liquid crystal material will assume a twisted structure in the interior of the cell, that is the director axis of the material will follow a helix as it progresses through the layer of liquid crystal material.
When an electrical potential is applied between the electrodes, the nematic structure of the liquid crystal material rotates or untwists. Light propagating within the cell has its plane of polarization altered to a degree depending upon whether the material is in its twisted or untwisted state. The polarizing filters on either side of the cell determine whether light is transmitted through the cell or blocked, depending upon their relative orientation, and the state (twisted or untwisted) of the liquid crystal material within the cell.
It is known that the time required for the liquid crystal material in the cell to become aligned or activated is a function of the magnitude of the electrical potential applied between the electrodes. The decay or relaxation time for the material is dependent upon the characteristics of the nematic material used in the cell. The magnitudes of the alignment and relaxation times become important where the time required for the state change of the material (from transmissive to opaque or vice-a-versa) determines its usefulness for a particular application.
Shuttering applications, such as those found in stereoscopic viewing systems, require that the cell have a fast response time, wherein the cell is dark (opaque) in one state, and transmissive (transparent) in the other. The prior art discusses attempts to decrease cell response times and to improve the contrast between the light transmissive and light blocking states.
Aftergut et al., in U.S. Pat. No. 4,143,947, teaches how to reduce the decay (relaxation) time in a 90 degree twisted nematic cell by adding a controlled amount of an optically active material (such as a cholesteric material) to the nematic liquid crystal material in the cell. Aftergut also indicates that a cell constructed so that the liquid crystal material (with the optically active additive) assumes a 270 degree twist in its structure realizes a further decrease in the decay time. However, a cell constructed according to the invention of Aftergut will have nonuniformities in image texture upon switching due to the tilt of the liquid crystal material director axes relative to the electrode surfaces of the cell.
Raynes, in U.S. Pat. No. 4,084,884, teaches that the sense of tilt of the director axes of the liquid crystal material at the electrode surfaces must be compatible with the direction of twist of the helical structure of the nematic material within the cell, in order to avoid nonuniform textures upon activation. Thus the tilt of the nematic material impacts the display characteristics of the cell, and its utility in shuttering applications, as they depend upon the contrast in transmissiveness which is achieved between the activated and relaxed states of the cell.
Bos, in U.S. Pat. No. 4,566,758, teaches how to fabricate a fast optical shutter having a reduced response time. The liquid crystal cell in Bos is a pi (.pi.) cell, in which the transparent plates are oriented antiparallel to each other. The switching between the cell states is fast because the motion of the liquid crystal material is limited.
A problem with the type of cell disclosed in Bos is that in production, surface defects show up as defects in the shutter capability because most of the switching behavior occurs near the plate surface. In addition, this type of cell uses a bias voltage which is applied in the "off" state to inhibit the relaxation of the cell material and to provide a neutral colored cell. The use of a bias voltage is undesirable in some applications owing to power restrictions or circuit complexity considerations. The invention of Bos also requires a tilt angle at the plate surfaces which is difficult to achieve in mass production.
What is desired is a liquid crystal device suitable for use as an optical shutter which has a fast response time, optimizes the contrast between the two cell states, does not require a bias voltage, and can be easily fabricated from standard materials by existing production methods.